Internal combustion engines have a cylinder block and at least one cylinder head which are connected to one another to form the cylinders. To control the charge exchange, an internal combustion engine requires control elements—generally in the form of valves—and actuating devices for actuating said control elements. The valve actuating mechanism required for the movement of the valves, including the valves themselves, is referred to as the valve drive. The cylinder head often serves to accommodate the valve drive.
During the charge exchange, the combustion gases are discharged via the outlet openings of the cylinders, and the charging of the combustion chambers, that is to say the induction of fresh mixture or fresh air, takes place via the inlet openings. It is the object of the valve drive to open and close the inlet and outlet openings at the correct times, with a fast opening of the largest possible flow cross sections being sought in order to keep the throttling losses in the inflowing and outflowing gas flows low and in order to ensure charging of the combustion chamber with fresh mixture, and an effective, that is to say complete, discharge of the exhaust gases. Therefore, the cylinders are also often provided with two or more inlet and outlet openings.
According to previous systems, the inlet ducts which lead to the inlet openings, and the outlet ducts, that is to say exhaust lines, which adjoin the outlet openings, are at least partially integrated in the cylinder head. The exhaust lines of the cylinders generally merge to form one common overall exhaust line, or else in groups to form two or more overall exhaust lines. The merging of exhaust lines to form an overall exhaust line is referred to in general and within the context of the present disclosure as an exhaust manifold, with that part of the overall exhaust line which lies upstream of a turbine arranged in the overall exhaust line being considered according to the disclosure as belonging to the exhaust manifold.
Downstream of the manifold, the exhaust gases are in the present case supplied, for the purpose of supercharging of the internal combustion engine, to the turbines of at least two exhaust-gas turbochargers and if appropriate to one or more systems for exhaust-gas aftertreatment.
An exhaust-gas turbocharger comprises a compressor and a turbine which are arranged on the same shaft, with the hot exhaust-gas flow being supplied to the turbine and expanding in said turbine with a release of energy, as a result of which the shaft is set in rotation. Owing to the high rotational speeds, the shaft is usually held in plain bearings. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the cylinders is obtained. If appropriate, a charge-air cooling arrangement is provided by means of which the compressed combustion air is cooled before it enters the cylinders.
Supercharging serves primarily to increase the power of the internal combustion engine. Here, the air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean effective pressure can be increased. Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. For the same vehicle boundary conditions, it is thus possible to shift the load collective toward higher loads, where the specific fuel consumption is lower.
The configuration of the exhaust-gas turbocharging often poses difficulties, wherein it is basically sought to obtain a noticeable performance increase in all rotational speed ranges. A severe torque drop is however observed in the event of a certain rotational speed being undershot. Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. In the case of a diesel engine, for example, if the engine rotational speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. This has the result that, toward lower rotational speeds, the charge pressure ratio likewise decreases, which equates to a torque drop.
Here, it would fundamentally be possible for the drop in charge pressure to be counteracted by means of a reduction in the size of the turbine cross section, and the associated increase in the turbine pressure ratio. This however merely shifts the torque drop further in the direction of lower rotational speeds. Furthermore, said approach, that is to say the reduction in size of the turbine cross section, is subject to limits because the desired supercharging and performance increase may be possible without restriction even at high rotational speeds, that is to say in the case of high exhaust-gas quantities.
One measure to increase the torque characteristic of a supercharged internal combustion engine, for example, is a small design of the turbine cross section and simultaneous provision of an exhaust-gas blow-off facility. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas mass flow exceeds a threshold value, then a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. This approach has the disadvantage that the supercharging behavior is inadequate at relatively high rotational speeds or in the case of relatively high exhaust-gas quantities.
The inventors herein have recognized the issues with the above approaches and provide a system to at least partly address them. In one example embodiment, a supercharged internal combustion engine comprises at least one cylinder head with at least two cylinders, wherein each cylinder has at least two outlet openings for discharging exhaust gases, at least one of which outlet openings is designed as an activatable outlet opening, and each outlet opening is adjoined by an exhaust line; a first exhaust manifold in which the exhaust lines of the activatable outlet openings of at least two cylinders merge, to form a first overall exhaust line which is connected to a turbine of a first exhaust-gas turbocharger; and a second exhaust manifold in which the exhaust lines of the other outlet openings of the at least two cylinders merge, to form a second overall exhaust line which is connected to a turbine of a second exhaust-gas turbocharger, wherein the first exhaust manifold and the second exhaust manifold are permanently connected to one another upstream of the two turbines via at least one connecting duct which cannot be closed off.
The torque characteristic of a supercharged internal combustion engine may furthermore be enhanced by means of multiple turbochargers arranged in parallel, that is to say a plurality of turbines of small cross section arranged in parallel, wherein turbines are activated with increasing exhaust-gas quantity.
A supercharged internal combustion engine of the present disclosure includes at least two exhaust-gas turbochargers arranged in parallel, wherein one turbine is designed as an activatable turbine which is acted on with exhaust gas, that is to say activated, only in the case of relatively high exhaust-gas quantities.
Here, it is sought to arrange the turbines as close as possible to the outlet, that is to say the outlet openings of the cylinder in order thereby firstly to be able to make optimum use of the exhaust-gas enthalpy of the hot exhaust gases, which is determined significantly by the exhaust-gas pressure and the exhaust-gas temperature, and secondly to ensure a fast response behavior of the turbochargers. In this connection, it is therefore fundamentally sought to minimize the thermal inertia and the volume of the line system between the outlet openings on the cylinders and the turbines, which may be achieved by reducing the mass and the length of the exhaust lines.
To achieve the above-stated aims, the exhaust lines of at least two cylinders are merged in a grouped manner in such a way that, from each of said cylinders, at least one exhaust line leads to the turbine of the first exhaust-gas turbocharger and at least one exhaust line leads to the turbine of the second exhaust-gas turbocharger.
According to the disclosure, the turbine of the first exhaust-gas turbocharger, that is to say the first turbine, is designed as an activatable turbine, and the outlet openings of the exhaust lines leading to said turbine are—correspondingly—designed as activatable outlet openings. Only in the case of relatively high exhaust-gas quantities are the activatable outlet openings opened, and the first turbine thereby activated, that is to say acted on with exhaust gas, during the course of the charge exchange.
In comparison with embodiments in which a single coherent line system is provided upstream of the two turbines, the above-described grouping, that is to say the use of two mutually separate exhaust manifolds, improves the operating behavior of the internal combustion engine, in particular at low exhaust-gas flow rates, since the line volume upstream of the second turbine, through which exhaust gas flows continuously, is reduced in size by this measure, which is advantageous, in particular improves response behavior, at low loads and rotational speeds, that is to say in the case of low exhaust-gas quantities.
A disadvantage of the above-described internal combustion engine is that the activatable turbine, in the deactivated state, is completely cut off from the exhaust-gas flow, that is to say no exhaust gas whatsoever is supplied to the deactivated turbine. This results from the use of a separate exhaust manifold and the non-opening of the activatable outlet openings in this operating state, that is to say in the case of small exhaust-gas quantities.
As a result of the lack of incident exhaust-gas flow, the rotational speed of the activatable turbine decreases considerably in the event of deactivation. The hydrodynamic lubricating film is depleted or breaks down entirely, such that the formerly purely liquid friction changes to mixed friction, if appropriate to predominant solid body friction. This increases wear, reduces the durability of the exhaust-gas turbocharger and may basically be regarded as a factor with regard to the susceptibility of the charger to faults. In this context, it may also be taken into consideration that the deactivated turbine, too, is subjected to vibrations and shocks during the operation of the internal combustion engine, wherein owing to the lack of a lubricating film, there is no damping of the components which move relative to one another.
Furthermore, it may be taken into consideration that the activatable turbine may be accelerated when it is activated. Even though the rotor of the activatable turbine has relatively low inertia owing to its size or the use of two turbines, the turbine responds only slowly upon activation. A torque demanded by the driver can therefore be provided only with a delay.
In contrast to the previous systems, according to which the two exhaust manifolds are completely separated from one another, it is provided according to the disclosure that the manifolds are connected to one another. For this purpose, at least one connecting duct is provided which cannot be closed off, that is to say is permanently open and which functions as an overflow duct.
Said connecting duct allows some of the exhaust gas to flow over from the second exhaust manifold into the first exhaust manifold even in the case of relatively low exhaust-gas quantities, such that the activatable turbine is acted on with exhaust gas via the second exhaust manifold and connecting duct even in the deactivated, that is to say shut-down state.
Here, there may be supplied to the activatable turbine via the connecting duct only such an amount of exhaust gas that the turbine shaft does not fall below a minimum rotational speed nT. Maintaining a certain minimum rotational speed prevents or lessens the depletion of the hydrodynamic lubricating film in the plain bearing of the shaft of the first charger. Consequently, the measure of supplying a small amount of exhaust gas to the activatable turbine even in the deactivated state has an advantageous effect in terms of the wear and the durability of the exhaust-gas turbocharger. Furthermore, the response behavior of the activatable turbine and of the supercharging as a whole is improved, because the activatable turbine is accelerated from a higher rotational speed when activated. A torque demanded by the driver can be provided comparatively quickly, that is to say with only a small delay.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.